Commercial services which employ a W-CDMA (Wideband Code Division Multiple Access) method which is included in communication methods called a third generation were started in Japan since 2001. Furthermore, a service with HSDPA (High Speed Down Link Packet Access) which implements a further improvement in the speed of data transmission using downlinks (a dedicated data channel and a dedicated control channel) by adding a channel for packet transmission (HS-DSCH: High Speed-Downlink Shared Channel) to the downlinks has been started. Currently, anHSUPA (High Speed Up Link Packet Access) method has also been suggested and studied in order to speed up uplink data transmission. The W-CDMA is a communication method which was determined by the 3GPP (3rd Generation Partnership Project) which is the organization of standardization of mobile communication systems, and the technical specification of the release 6 has been being organized currently.
In the 3GPP, as a communication method different from the W-CDMA, a new communication method having a wireless section, which is referred to as “Long Term Evolution” (LTE), and a whole system structure including a core network, which is referred to as “System Architecture Evolution” (SAE), has been studied. The LTE has an access method, a radio channel configuration, and protocols which are different from those of the current W-CDMA (HSDPA/HSUPA). For example, while the W-CDMA uses, as its access method, code division multiple access (Code Division Multiple Access), the LTE uses, as its access method, OFDM (Orthogonal Frequency Division Multiplexing) for the downlink direction and uses SC-FDMA (Single Career Frequency Division Multiple Access) for the uplink direction. Furthermore, while the W-CDMA has a bandwidth of 5 MHz, the LTE can have a bandwidth of 1.25/2.5/5/10/15/20 MHz. In addition, the LTE uses only a packet communication method, instead of a circuit switching method which the W-CDMA uses.
According to the LTE, because a communication system is constructed by using a new core network different from a core network (which is called General Packet Radio System GPRS) which complies with the W-CDMA, the communication system is defined as an independent radio access network which is separate from a W-CDMA network. Therefore, in order to distinguish from a communication system which complies with the W-CDMA, in a communication system which complies with the LTE, a base station (Base station) which communicates with a mobile terminal UE (User Equipment) is called eNB (which may be referred to as E-UTRAN NodeB or eNodeB in some cases), a base station control apparatus (Radio Network Controller) which performs exchange of control data and user data with a plurality of base stations is called an aGW (Access Gateway). This communication system which complies with the LTE carries out point-to-multipoint (Point to Multipoint) communications, such as a multicast and broadcast type multimedia service called an E-MBMS (Evolved Multimedia Broadcast Multicast Service), and also provides a communication service such as a unicast (Unicast) service for each mobile terminal among a plurality of mobile terminals. In the case of the LTE, because no dedicated channels (Dedicated Channel and Dedicated Physical Channel) destined for each mobile terminal exist in transport channels and physical channels, transmission of data to each mobile terminal is carried out by using a shared channel (Shared channel), unlike in the case of the W-CDMA.
When data transmission occurs in an uplink or a downlink, scheduling which enables communications between the base station and the mobile terminal is carried out for the uplink or the downlink. For example, in the downlink scheduling, the base station allocates radio resources according to both the size of data which have occurred and the communication path quality to the mobile terminal, and sets up a modulation method and an error correcting code method (MCS: Modulation and Coding scheme) according to target quality and a target data speed. In the uplink scheduling, when transmission data destined for the base station occur in the mobile terminal, the mobile terminal transmits a signal (an uplink scheduling request SR: Scheduling Request) with which to make a request for allocation of uplink radio resources, and the base station allocates uplink radio resources to the mobile terminal in response to the signal. Control signals used for such scheduling control which enables communications between the mobile terminal and the base station via a radio link include an upper layer signal, such as an “L3 control signal” (Layer3 control signaling or L3 message), and a signal which is called an “L1/L2 control signal” (Layer1/Layer2 control signaling). An L3 control signal is typically notified from an upper layer like an RRC layer at the time of initial transmission including the time of occurrence of a call connection (RRC Connect), and is used, via a downlink, for an uplink channel setup, a downlink channel setup, or allocation of radio resources. In contrast, an L1/L2 control signal is frequently exchanged between the mobile terminal and the base station in both an uplink and a downlink. As an uplink scheduling request signal with which the mobile terminal makes a request of the base station for allocation of radio resources via an uplink, the mobile terminal uses an L1/L2 control signal. Also at the time of changing the radio resources irregularly according to a change in the data size or a requirement of desired quality of the communication path, including the time of occurrence of a call connection and the time of continuation of a call connection, the mobile terminal uses an L1/L2 control signal. L1/L2 control signals include an Ack/Nack with which the base station or the mobile terminal answers whether it has received data correctly when receiving the data by way of an uplink or a downlink, and quality information CQI (Channel Quality Indicator) indicating the quality of received data or the quality of the communication path.
A core network according to the LTE is a network with packet connection, and user data, including real time data, such as voice data, are altogether packetized. In a case of general transmission of packet data, no real-time nature is required of the data, and the speed of the data which are transmitted and received can vary irregularly according to the description of the data. In contrast, because real time data, such as voice data, have to be reproduced in real time by a communications partner even if they are packetized, data having a predetermined size can occur at fixed time intervals. Therefore, in allocation of radio resources which results from scheduling, different scheduling methods are needed when carrying out general communications of packet data and when carrying out communications of real time data such as voice data. For data, such as general packet data, whose speed is varied according to the description of the data and which have to comply with high speed communications, a dynamic scheduling (dynamic scheduling) method of being able to change the settings of the radio resources dynamically according to the communication path quality and the data speed (the data size) every TTI (=1 ms) is used. On the other hand, for communications, such as communications of voice data, in which a real-time nature is required and data having a predetermined size occur at fixed time intervals, a persistent scheduling (Persistent scheduling) method of being able to allocate radio resources at fixed time intervals and continuously is used because the communications are carried out at a low speed and the data have either of one or more determined sizes. A modulation method and error correction conditions (MCS) can be allocated at fixed time intervals and continuously according to both the size of data which can occur and predetermined target quality. Nonpatent reference 1 discloses, as a merit of the persistent scheduling, large reduction in the volume of traffic of L1/L2 control signals transmitted between a mobile terminal and abase station because the base station does not have to carry out a setup and update of radio resources by using an L1/L2 control signal and does not have to report a received data quality report (CQI) every TTI (=1 ms) after notifying allocation of radio resources and settings of MCS to the mobile terminal by way of an L3 control signal at the time of initial transmission.
However, because the data rate of voice data which are actually communicated in a network varies at an arbitrary timing due to the user's talk quality or operation during talk, the base station actually has to perform a setup and an update of radio resources in the course of voice communications by using an L3 control signal or an L1/L2 control signal. In voice communications which comply with the LTE, it is expected that a method called AMR (Adaptive Multi Rate), which is used, as a standard voice codec, by the W-CDMA of the 3GPP, will be adopted. The AMR method which is assumed, as a standard method, by the 3GPP includes a method which is called narrowband (Narrow band), and a method which is called wideband (Wide band). The narrowband AMR is a coding method which is based on that a voice is sampled at a frequency of 8 kHz. In contrast, the wideband AMR is a coding method which is based on that a voice is sampled at a frequency of 16 kHz, and supports higher-speed multimedia data and is aimed at implementation of high-rate and high-quality voice communications. In nonpatent reference 2, FIG. 5 is a figure showing an operation at a time of communicating packetized voice data (VoIP data) via an uplink after the voice data are compressed by using the narrowband AMR. As shown in the nonpatent reference 2, the state of voice communications which use AMR for compression encoding is divided into the following three states: a transient state (transient state), a talk state (a talk spurt, a talk time, or a talk period), and a silent state (a silent period or a VOX period). In the transient state and in the talk spurt, data are updated every 20 milliseconds. In the silent state, if a section in which voice data do not occur is long, background noise data (SID) are updated every 160 milliseconds. A transition to this state occurs at an arbitrary timing. Because there is a high possibility that the communication quality state changes due to a transition to the silent state, it is necessary to change the radio resources and the settings of MCS by way of a control signal in the course of the transition to the silent state. Because, in a case of carrying out persistent scheduling at a time of communications of real time data such as voice data, a control operation of updating the data at fixed time intervals and changing the data rate and the time intervals at which data occur in the course of the communications is performed, the issue of omitting the useless control of communications between the base station and the mobile terminal while maintaining the communication quality in the course of the communications, and simplifying the resource management in the scheduling, thereby reducing the operating load on both the base station and the mobile terminal, and the issue of how to respond to the real-time nature of data must be addressed.
In nonpatent reference 3, as to a persistent scheduling method for use in voice packet data communications via an uplink, a plurality of suggestions by several companies are compared. The nonpatent reference 3 discloses that all the companies suggest that in a state transition between a silent state and a talk state which temporarily occurs at a time of voice communications with AMR, a re-setup and a change of radio resources have to be carried out between a mobile terminal and a base station by using an L1/L2 control signal or an L3 control signal. However, the nonpatent reference 3 only lists problems that a communication delay and a waste of the resources can occur when an overhead of a control signal or an receiving error of a control signal occurs for each suggestion, but does not disclose any concrete solution of “the problems to be solved by the invention” as shown in the specification of the present invention, and any suggestion as to “the advantages of the invention” as shown in the specification of the present invention.
As the method of allocating radio resources which are used at a time of data communications according to the LTE, there are a radio resource allocation method which is called “localized” (localized) and a radio resource allocation method which is called “distributed” (distributed) (nonpatent reference 4). FIGS. 6(a) and 6(b) are figures showing a method of dividing a time-frequency region which a base station can use into a plurality of blocks on the frequency axis and on the time axis, and allocating them to mobile terminals. Each divided block unit is called a resource unit (RU: Resource Unit) in the case of an uplink, and is called a resource block (RB: Resource Block) in the case of a downlink. FIG. 6(a) shows an example in which radio resources are allocated in a localized fashion on the time and frequency axes, and FIG. 6(b) shows an example in which radio resources are allocated in a distributed fashion on the time and frequency axes. As shown in FIG. 6(a), the localized allocation is a method of allocating radio resources having one or more continuous frequency bands on the frequency axis at the same timing. In contrast, in the distributed allocation shown in FIG. 6(b), two or more radio resources which are separated from one another (=distributed) are simultaneously used on the frequency axis. In the 3GPP, localized allocation as shown in FIG. 6(a) has been studied for an uplink and localized radio resource allocation has been studied for a downlink, and distributed radio resource allocation as shown in FIG. 6(b) has been studied.
Nonpatent reference 5 discloses a radio resource allocation method of, as to persistent scheduling, dividing the interior of one radio resource block into parts with a plurality of frequencies for a downlink and then distributing and allocating radio resources used for a mobile terminal to the plurality of divided parts of the resource block, and making a hopping of one radio resource frequency to allocate for an uplink. However, the nonpatent reference does not disclose any solution of “the problems to be solved by the invention” as shown in the specification of the present invention.    [Nonpatent reference 1] 3GPP contributions R2-061920    [Nonpatent reference 2] 3GPP contributions R1-070333    [Nonpatent reference 3] 3GPP contributions R2-070283    [Nonpatent reference 4] 3GPP TR25.814V7.0.0    [Nonpatent reference 5] 3GPP contributions R1-070098
In scheduling for use in conventional user data communications, dynamic scheduling of, by, generally, using an L3 control signal or an L1/L2 control signal, performing a setup of a modulation method and making settings of conditions (MCS) of an error correcting code, as needed, and also performing allocation of radio resources plays a predominant role. On the other hand, in recent years, for real time data, such as voice data, which occur at fixed time intervals and continuously, a scheduling method, which is called persistent scheduling, of performing a setup of a modulation method and making settings of conditions (MCS) of an error correcting code, and also performing allocation of radio resources in advance according to the regularity of data occurrence at the time of initial transmission has been suggested. However, because the data rate and the data generation time intervals of real time data, such as actual voice data, change in the course of communications according to the quality of the voice talk and the user's operation (a silent state), control according to these changes is needed in the course of communications. A challenge is therefore to change the allocation of radio resources and the MCS according to the data rate and the quality of data which vary at an arbitrary timing in order to make effective use of the radio resources and maintain the communication path quality, and to also reduce useless resource allocation which is caused in that case, reduce the amount and frequency of control signals which are generated in the course of communications, and reduce the system load. Another challenge is to provide a radio resource allocation method which makes it easy to stabilize the communication quality in order to reduce the occurrence of control signals in the course of communications. A further challenge is to reduce the delay which is caused by an overhead or a receiving error of a control signal transmitted between the base station and the mobile terminal, thereby reducing the delay to a minimum so that the data can be reproduced in real time.
The present invention is made in order to solve the above-mentioned challenges, and it is therefore an object of the present invention to provide a data communication method, a communication system, and a mobile terminal which, when performing persistent scheduling, can not only cope with the data rate and quality which vary at an arbitrary timing, and but also perform resource management which enables effective use of the radio resources in a whole system. It is another object of the present invention to provide a data communication method, a communication system, and a mobile terminal which change allocation of radio resources and MCS in the course of communications according to a change in the data rate and a change in the data generation time intervals which can occur at an arbitrary timing, and also reduces useless resource allocation, which can reduce the amount of occurrence and the frequency of an L3 control signal or an L1/L2 control signal which occurs in the course of communications, and which perform scheduling which makes it easy to stabilize the communication quality. It is a further object of the present invention to provide a data communication method and a communication system which perform scheduling with a delay, such as an overhead of a control signal transmitted between a base station and a mobile terminal, which does not affect real-time reproduction of data, and the mobile terminal.